It has long been known to manufacture and distribute pressure-sensitive adhesive stock for labels by providing a layer of face or facestock material for the label or sign backed by a layer of pressure-sensitive adhesive which in turn is covered by a release liner or carrier. The liner or carrier protects the adhesive during shipment and storage. The liner or carrier also allows for efficient handling of an array of individual labels after the labels are die-cut from the layer of facestock material and up to the point where the individual labels are dispensed in sequence on a labelling line. During the time from die-cutting to dispensing, the liner or carrier remains uncut and may be rolled and unrolled for storage, transit and deployment of the array of individual labels carried thereon.
In many label applications, it is desirable that the facestock material be a film of polymeric material which can provide properties lacking in paper, such as durability, strength, water resistance, abrasion resistance, gloss and other properties. Heretofore, facestock material of thicknesses greater than about 3 mils have been used in order to assure dispensibility in automatic labelling apparatus. However, it is desirable to reduce the thickness or "down-gauge" the facestock material in order to attain savings in material costs. Previously, such reduction in label thickness has resulted in reduced stiffness and the inability to dispense the labels in a reliable commercially acceptable manner using automatic machinery. One standard of dispensibility requires dispensing rates of about 200 units per minute for an average size label of about 3-1/2".times.5" and less than about 5% failure. In some applications the customer may require an even lower fail rate, e.g. less than 1%.
Failure to reliably dispense is typically characterized by the label following the carrier around a peel plate without dispensing or "standing-off" from the carrier for application to the substrate. Such failure to dispense is believed to be associated with excessive release values, measured as disclosed below, between the label facestock material and the liner, and the maximum workable release level is dependent upon the stiffness of the facestock. Failure to dispense may also be characterized by a folding of the label due to lack of label stiffness at the dispensing speed as it is transferred from the carrier to the substrate. Another particular need in many labelling applications is the ability to apply polymeric-film labels at high line speeds, since to increase line speed has obvious cost-saving advantages.
In the prior art, one useful polymeric material is biaxially-oriented polypropylene ("BOPP") which is relatively inexpensive and dispenses well but is not usable on squeezable bottles and other flexible substrates. Such films tend to have sufficient stiffness for dispensing, but they also have relative high tensile modulus values, e.g. 100,000 psi or greater, which results in unacceptable conformability characteristics. Another useful material is unoriented blown-film polyethylene that is also relatively inexpensive and is usable on squeezable bottles and the like, but is difficult to die cut consistently and dispenses very poorly. In this instance, it is believed that the stiffness is not sufficient for reliable dispensing. In general, in the prior art, polymeric-film labels have not been entirely satisfactory in die-cut label applications, particularly those involving polymeric-film materials less costly than "vinyl", i.e. polyvinyl chloride (PVC). For example, down-gaging of polymeric-film labelling stock for improved economy has been inhibited by dispensing problems and the speed of label application lines has been limited when applying polymeric-film labelling stock less costly than PVC but still otherwise suitable for die-cut labels used on squeeze bottles or other flexible or deformable substrates. Polymeric-film label stock that is otherwise economically attractive and that is suitable for high speed dispensing has not lent itself to the labelling of flexible substrates, and polymeric-film stock that is otherwise economically attractive and that is suitable for labelling of flexible substrates has not been dispensable at high line speeds.
Related conformability problems have been encountered in respect to rigid substrates such as provided by glass bottles, especially clear glass bottles. More particularly, the suggested use of biaxially oriented film to label a glass bottle was unsuccessful due to the tendency of the relatively stiff label to bridge surface depressions resulting from the bottle forming process and to result in an undesirable surface appearance simulating a trapped air bubble. This has impeded the use of pressure-sensitive adhesive labels to replace prior glass bottle labelling techniques such as ceramic ink directly bonded to the bottle surface during the glass blowing process. Such ceramic ink techniques are environmentally undesirable due to objectional ink components and the contamination by the ink of the crushed bottle glass in recycling processing.
The heat-set labels contemplated by the methods of the present invention and the die-cut label applications to which the present invention relates are to be contrasted with shrink-films, consisting of stretched, unannealed films, sometimes used in sleeve-labelling applications wherein a sleeve or wrap of shrink film is placed around the circumference of a bottle or can or like container and heated to cause it to shrink into light, surrounding engagement with the container. Examples of the latter are found in U.S. Pat. No. 4,581,262 and 4,585,679. The tendency to shrink causes such film to tend to withdraw from any borders, tending to leave exposed adhesive, a particular disadvantage in die-cut label applications since exposed adhesive is unsightly and tends to catch dust.